I’m not sure I understand. Generally, downscaling an image doesn’t imply any lack of fidelity. The problem comes when you upscale a small image.
When you say “but the scalled 160x120 png is not very real.kind of lacking fidelity”, do you mean you are upscaling your 160x120 image to 2X or downscaling your 320x240 image to 0.5X ? In the first case, of course it’s going to look bad : you need to load the 320x240 image instead. In the second case, there shouldn’t be any problem.
Another strange thing you said is “scalling it to 160x120 using the same texture.” What do you mean by that ? Because your two images are going to be different textures, CCTextureCache takes care of that.
Last but not least “memory use can be very sensitive in a game” : you have to be careful, but if you don’t go overboard and use assets depending on the device screen size and use the easy cocos2d optimizations, you shouldn’t have much problems.